The present invention relates to a linear motion drive means for cables and the like.
Various means have been devised to move a cable in a direction lying along its axis. It will be understood that the term "cable" when appearing in the phrase "driven cable", as it is used throughout this specification shall include, in addition to its conventional meaning, such items as lengths of pipe, thin-walled tubes, and solid rod or bar stock. Some of these prior art driving devices are objectionable because they cause or allow rotation of the cable. While this may be desirable for some uses, there are other applications where such rotation cannot be tolerated.
Other drive devices employ rollers, belts or chains which merely lie along and engage the exterior of the driven cable transmitting frictional drive forces. Such drive means are subject to slippage which is objectionable, not only because the cable is not positively driven, but also because the slippage increases the amount of localized wear on the drive means and/or the cable which can result in further slippage or localized breakages. Additionally, these devices are objectionable for failing to adequately distribute the drive forces since they have only point or limited line contact with the surface of the cable.
The present invention is directed to a linear cable drive which overcomes these deficiencies. More particularly, the present invention is directed to a linear cable drive comprised of a plurality (preferably three or more) of drive assemblies each of which includes a drive cable or belt which is helically wrapped around the driven cable. If these elements were then, to merely be driven, they would rotate, as well as translate, the driven cable. In order to prevent this undesired rotation, the drive assemblies are rotated about the longitudinal axis of the driven cable in a direction, and at a rate, to unwind the leading end of the drive cable and cancel out the rotational forces.
Two devices known in the prior art which are similar in some respects to the present invention are shown in U.S. Pat. Nos. 2,789,687 and 3,265,269. Due to the generally unstable configurations of these drive assemblies, each of these devices requires a guide tube which has a close tolerance with respect to the pipe or cable which is to be driven. Accordingly, these drive systems are incapable of handling a discontinuity which is either smaller or larger than the base peripheral dimension. It should be noted here that merely providing clearance and a stable (self-balancing) configuration would be inadequate to accommodate the discontinuity. The same length of drive cable engaging a larger or smaller peripheral dimension would cause the discontinuity to undergo a rotational twisting. As this discontinuity is apt to be a splice or connector between adjacent cable lengths, such a twisting would be particularly undesirable and potentially harmful.
The present invention provides a means to accommodate discontinuities in cable peripheral dimension while maintaining a linear, non-twisting drive relationship. An additional advantage of the present invention is that this drive means is provided with means to facilitate the initial infeed of the cable. By detaching certain elements, the helical drive belts can be unwound to enlarge the central space so that the cable may be more easily inserted, possibly even permitting room for insertion of a hand and arm, depending on the size of the unit.
A further advantage of the drive means of the present invention is its versatility. A particular unit can handle a wide range of cable diameters. Further, additional drive force can be provided by adding assemblies to a particular unit, stacking units end to end or interdigitating units.
A further feature of the present invention is that the concept can be utilized to form a holding device in lieu of a drive mechanism.
These and other features, advantages and objects of the present invention will be better understood after reading the following specification in conjunction with the accompanying drawings.